(57) Apparatus for producing moulded castings in polymer materials includes reservoirs (6, 8, 90) for constituent materials (such as bis-phenol A based expoxide resin, acid anhydride or polyisolcyanate hardener, pigments, dilutents and catalysts), metering devices (11, 13, 92) arranged to control outflow from the reservoirs interconnected (70) to proportion the outflows and mixing means (14) arranged to mix together the outflows. The mixed material is supplied to an accumulator (16) discharging to a cavity mould (2) positioned within an autoclave (4). The autoclave interior and the interior of the cavity mould (2) are respectively connectable alternatively to a source of air at superatmospheric pressure (26), or to a sub-atmospheric pressure exhaust (56), or a further sub-atmospheric pressure exhaust (60), or to a source (40) of purging gas (such as sulphur hexafluoride). In operation, the cavity mould (2) with an electrical component in place is loaded into the autoclave (4), exhausted and purged in two stages and then filled with the polymer materials from the accumulator (16). Polymerisation is then effected by pressurizing and heating the autoclave (4) to encapsulate the electrical component. The gas pressures within the cavity mould (2) and the air pressures the autoclave (4) are kept in approximate balance during exhausting and purging to contain the pressure differential across the cavity mould wall within acceptable limits.